Carburization is a conventional method for case hardening of various ferrous articles, such as steel components. In the typical gas phase carburization technique, a gas atmosphere is utilized which has the capability of transferring carbon to the surface of the steel article being treated, such that the carbon is adsorbed onto the surface of the article and then diffused at appropriate temperatures into the surface zones of the article. Various carbon donating atmospheres have been used in the past including endothermic atmosphere produced from the combustion in a heated catalytic retort under partial oxidation conditions of a hydrocarbon with air to produce a mixture of carbon monoxide, hydrogen and nitrogen. Typically, endothermic atmospheres are produced external to the carburizing furnace and blended with additional hydrocarbon enriching gas prior to entering the furnace.
It is also known to synthetically produce such an endothermic atmosphere within the carburizing furnace by blending methanol, nitrogen and a hydrocarbon and subjecting the mixture to high temperatures. It is theorized that the carbon monoxide acts as a shuttle for carbon from a high temperature heat source in the furnace to the surface of the ferrous article being treated. The source of the shuttled carbon is the enriching hydrocarbon gas. Such hydrocarbon is cracked under conditions of high temperature in the carburization furnace on the heat source or radiant tubes which are at a higher temperature than the articles. Water, present as a by-product in the carburization reactions, is believed to combine with the carbon cracked on the high temperature heating surfaces of the carburization furnace to form carbon monoxide and hydrogen. As the carbon monoxide contacts the ferrous articles to be carburized, the carbon monoxide reacts with hydrogen to deposit carbon on the article and result in water as a by-product. Therefore, the presence of carbon monoxide acts to shuttle the carbon of the cracking enriching gas from a high temperature surface in a carburizing furnace to the lower temperature surface of the articles being carburized, wherein the carbon monoxide disassociates to form carbon and oxygen, the latter of which reforms with available hydrogen to form water. It can be seen that in order to accelerate the carburizing effect, carbon monoxide must be readily available to shuttle carbon from the high temperature surface to the carburizing article. It is equally important to provide a carbon source consisting of the enriching gas in order to replenish the carbon utilized from carbon monoxide during the carburization and to allow the water of the carburizing reaction to reach the site of the cracking hydrocarbon so as to form carbon monoxide with such carbon.
This theory of carbon shuttling is set forth in an article by Kaspersma and Shay entitled "A Model For Carbon Transfer In Gas-Phase Carburization of Steel" presented in the Journal of Heat Treating, Vol. 1, No. 4, at page 27.
It is also known to utilize a methanol and enriching gas atmosphere without nitrogen in at least one stage of the carburization process as set forth in an article by Peartree entitled "Two-Step Accelerated Carburizing Shortens Cycle, Saves Energy" presented in Heat Treating, July 1981, page 36. The use of dissociated methanol and enriching gas undiluted with nitrogen in a continuous belt furnace is also discussed. The ratio of enriching gas to methanol was similar to that employed in a conventional carburizing furnace. Accelerated carburizing rates were observed with both techniques. The two-stage process set forth in the article wherein a pure methanol-methane atmosphere is initially used in one stage, while a synthetic endothermic atmosphere is used in a second stage of a carburization process is also the subject of U.S. Pat. No. 4,306,918.
In U.S. Pat. No. 4,317,687, a process for carburizing ferrous metal articles is set forth wherein nitrogen, ethanol and water are injected into the furnace with or without a hydrocarbon enriching agent, such as propane, to produce the carburizing atmosphere.
Patents of additional interest to the carburization art include U.S. Pat. No. 4,145,232 which is directed to a carburizing atmosphere in a furnace wherein the hydrocarbon is maintained in a precise concentration Z.sub.A below the level of 10% so as to minimize the amount of carburizing gas necessary, and U.S. Pat. No. 4,322,255 which is directed to a carburizing atmosphere wherein the atmosphere is measured in the carburizing furnace and the hydrocarbon content is controlled in the range of 0.2 to 30%.
When endothermic, synthetic endothermic and other carburizing atmospheres are introduced into a one zone or open furnace, the rate of carbon monoxide formation is relatively high because of the high temperature of the radiant heat tubes or other heat sources of the carburizing furnace, which high temperature surfaces are readily accessible and favor carbon monoxide formation from the enriching hydrocarbon. However in a continuous rotary retort furnace or any furnace having discrete zones for carburizing and for the heat sources where each is isolated from the other, the carburizing atmosphere that comes in contact with the articles to be carburized does not enjoy the advantage of the high temperature heat source surfaces and therefore the rate of carbon monoxide formation is reduced and carburization of the articles is retarded. Practitioners in the prior art have attempted to remedy this result by further enriching the carburizing mixture to the entire furnace to enhance the formation of carbon monoxide at the lower temperatures of the furnace retort by increasing the proportion of hydrocarbons to crack at the lower rate of conversion. This attempt to maintain or enhance carburization creates the sooting problem in the heat source zone because the enriched carburizing mixture also contact the radiant heat tubes of the furnace and deposits undesired carbon thereon at the higher rate that the higher concentration of enriching gas dictates.
The present invention overcomes this problem of sooting and the ensuing furnace inefficiency and downtime by the method set forth below which reduces sooting and enhances carburization by using two techniques which individually would aggreviate these problems.